Digital disks such as compact disks (CDs) or digital versatile disks (DVDs) may include a multiple layer process for manufacture. The disk may include a first layer having a pattern of microscopic pits or readout information applied thereto. The pattern of pits includes the digital readout information for the disk. The layer may have one or more additional layers applied to it. The readout surface may include a single or dual-layer readout surface that may include a sputtered-on reflective (e.g., aluminum) layer. These one or more additional layers may be bonded to the first layer or subsequent layers to form a completed optical disk.
Each layer may include a different material with its associated material properties. Since these layers are typically polymeric, large mismatches can occur due to differences in the coefficients of expansion/contraction of the materials. These polymeric materials are generally molded and shrink at different rates when cooled. The shrinkage can be accounted for by compensation in the size and tolerance of the molds used to produce the disk layer. However, designing and modifying molds is costly and difficult. The level of complexity is increased greatly by attempting to account for mold shrinkage for parts made from different materials, especially when these materials need to line up or fit together.
When two disks are being bonded, mismatches in alignment relative to the center hole (radial runout) of the disks needs to be controlled. This becomes especially difficult when the disks are formed from different materials.